My son has dehydration. should i be concerned?

Yes: It more important to figure out why does he have dehydration. It can be sever and lead to sigificant short and long term problems.

Context important: Dehydration isn't good, but the context of your question determines how concerned you need to be: son's age?, level of activity?, medical impairments (concurrent heart or kidney disease)?, hospitalized/institutionalized/independent living?, ability to self-rehydrate or need iv?, preventable factors?, other problems (fever, vomiting, pain)?, ongoing medications (is he on diuretics, diet pills)?